The present invention relates, in general, to the field of direct current ("DC") motor control circuits. More particularly, the present invention relates to an integrated circuit ("IC") "H" switch, or H-bridge, motor control circuit incorporating a boost capacitor for the low-side metal oxide semiconductor ("MOS") transistor switching devices in the H-bridge in which the characteristics of the respective capacitors are closely matched to that of the associated switching device.
The DC low voltage fractional horsepower motor is a standard for applications that require bidirectional shaft control such as cassette tape drives, automobile power windows, seats, door locks or windshield wipers, robotic mechanical controls, small industrial machines etc. As such, H-bridge circuits have been developed in lieu of mechanical reversing gears and linkages for reversing the motor supply voltage. In an H-bridge, opposing top and bottom switching devices (bipolar, Darlington or MOS transistors for example) couple the DC motor input leads to a voltage source and circuit ground to control the motor direction by switching the voltage polarity, which, in turn, switches the motor's shaft rotation either clockwise or counter-clockwise.
Bipolar transistors are generally not satisfactory in these applications due to their relatively large base drive requirement and Darlington transistors exhibit a large forward voltage drop across the devices which reduce voltage to the motor. For these reasons, MOS transistors are often utilized in H-bridge applications as disclosed in U.S. Pat. No. 4,454,454 issued Jun. 12, 1984 for "MOSFET "H" Switch for a DC Motor".
Still other H-bridge circuits have utilized MOS transistors in order to control the current through the load in addition to switching voltage polarity. In this regard, the low-side switching devices function as current generators to force a defined current through the load. However, after each transition, the current generators take a finite time to reach a steady-state condition. In order to reduce this time and speed the overall system operation, an additional charge, through the use of an inverter and associated boost capacitor, may be added to the gate of the low-side switching devices. However, for optimal performance, there must be a good match between the boost capacitors and the input capacitance of the switching transistors. This is especially difficult to achieve in integrated circuit H-bridges due to the inherent variations in processing technology.